update cardinality text for omitted optional fields

[lumjjb]

As a follow up based on conversations on the mailing list (https://lists.spdx.org/g/Spdx-tech/topic/100823660), and on today's tech call 2023-08-22, (minutes) - . This PR is to add additional text to communicate that omission of fields with optional cardinality should be treated as signaling NOASSERTION.

[bact]

Since the proposal here has been discussed in the Tech call already.

Do we like to also have this proposed text:

Unless specified otherwise, omission of an optional field should be interpretted as signaling NOASSERTION.

in the Conformance section of post-v3.0 version, like v3.0.1, as well?

@lumjjb @goneall

[goneall]

Discussed on the 15 Oct 2024 tech call - there were a couple concerns with documenting this for the 3.0 spec:

• NOASSERTION is declaring a known unknow and we may loose the semantics of this being an intentional declaration
• NOASSERTION in 3.0 is actually for an Object property with a range of ELEMENT

Leaving this issue open for further discussion.

[bact]

• Related issue: Stable Semantics over Oprimization of Representation? spdx-3-model#896